the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 25 Jun 2026
Temporal variability in offshore Fe fluxes in the Peruvian oxygen minimum zone across an El Niño termination
Abstract. Offshore fluxes of Fe in the Peruvian oxygen minimum zone are sensitive to seawater redox state, and potentially modulated by the El Niño-Southern Oscillation. However, observational data on what controls such temporal variation in offshore Fe fluxes remain scarce. Here, we report seawater concentrations of dissolved Fe (dFe), particulate Fe (pFe), and their isotopic compositions (δ56dFe and δ56pFe) from two transects at 12° S and 14° S across the Peruvian shelf. Our data show clear shelf-to-slope dFe and pFe plumes with negative δ56Fe signatures. Maximum Fe concentrations at 12° S decreased from 13 to 4 nmol kg−1 (dFe) and 122 to 13 nmol kg−1 (pFe) over a month across an El Niño event termination. We link these variations to an intensifying Peru-Chile Undercurrent which enhanced oxygen supply and thereby weakened sedimentary reductive Fe effluxes. Off shelf, elevated δ56dFe and low δ56pFe in anoxic waters are attributed to organic ligand binding and authigenic Fe formation.
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Status: open (until 24 Sep 2026)
- RC1: 'Comment on egusphere-2026-3486', Anonymous Referee #1, 24 Aug 2026 reply
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RC2: 'Comment on egusphere-2026-3486', Anonymous Referee #2, 24 Aug 2026
reply
The manuscript of Chen et al. investigates iron transfert across the slope offshore Peru during spring 2017. Six vertical profiles of dissolved and particulate iron concentrations with their associated isotopic signature δ56 are described and seem of good quality. Six propositions are done along the manuscript. They are presented by order of importance starting with 1) a plume of sediment-derived dFe originally from the inner shelf extends over more than 100 km offshore ; 2) this plume containt also significant amount of pFe produced in the inner shelf; 3) interannual variations compared to previous cruise (GP16) are due to El Nino event captured in 2017; 4) monthly variations between two legs of the cruise are due to Peru-Chile Undercurrent variations 5) in one station, the plume is not visible due to eddy transport 6) a previous proposition from John et al. 2018 to explain a deeper dFe plume over 1000 km is wrong because others studies have demonstrated that non reductive dissolution of sediment are important persistent sources of dFe.
I have major reservations about the interpretations proposed. Propositions 3 and 4 are presented as hypothesis. Proposition 1 and 5 are not demonstrated which is a particular shame since the demonstration of proposition 1 should be the main goal of the manuscript. The demonstration of proposition 2 presents several contradictions or approximation (in particular with the figure 5). Finally the proposition 6 does not appear to be justified on the basis of the results presented in manuscript. I recommand the authors to reconsider all the discussion focusing on the propositions 1 and 2 but with a solid argumentation.
I will further focus only on the main proposition, proposition 1, whose authors do not discuss its validity using expressions as « striking feature » l. 169 ; « were evidently transported » l.192, « was also evident » l.203 instead. However, an important discussion is required to state that 1a) the dFe of the plume is sediment-derived and 1b) it extend over « 100 km » (l.127) « to the open ocean » (l.192).
To estimate the plume extension, I visualized their dataset of dFe (from Zhu et al 2021) and compared it with GP16 dataset that supposedly missed the plume.The occurrence of a original « 100 km » long plume is not evident… station TM 58 and TM66 seems not related with the plume of dFe event during El nino 2017 while a plume above the slope was also detected in GP16. Station TM55 and TM56 does show a original feature, a more pronounced plume of dFe at about 150m depth not directly connected with the bottom sediment, but it does not extend over 100 km. The high interest of the presented dataset is the significant signal of ligth δ56dFe < -1 ‰ (only visible on TM55 and TM56) that deserves to be discuss based on a solid argumentation (note that, surprisingly, no isotope data at high dFe concentration (> 5nM) is available from GP16). I recommand to the author to only focus on these stations to build a strong discussion about the extension and the origin of the plume.
How to build a solid argumentation concerning the plume origin ? First, the proposition have to be sustained clearly by the dataset. If dFe is transported from the sediment you should be able to draw a mixing line (δ56Fe = f(1/dFe)) along the pynocline of the water sediment interface releasing most of the dFe, in the inner shelf. The figure does not need to be perfect, but it will give the opportunity for the reader to get his/her own appreciation. Second, the results not perfectly consistent with the proposition requires alternative explanations. Why no plume is visible before station TM54 (Figure R1) ? Why very light isotope signal often observed above the OMZ if it is only a signature of sediment effluxes ? If light isotope can origin from another source, why this alternative source not significant in TM55 and TM56 ? Why no plume visible in station TM66 ? only the last question gets an answer in the current manuscript but without the required proves (proposition 5). Third, the proposition should also be defended face to alternative common explanations not even discussed in this version of the manuscript : as micronutrient, is the release of dFe during organic matter remineralisation significant in your system ? probably since you in a very productive area, can you estimate the dFe produced during remineralisation ? as a redox sensitive species, why the in situ reduction of Fe oxides (from atmospheric deposition, or offshore surface transport) negligeable ? if reductive dissolution of Fe oxides occurs in the top of the OMZ, the isotopic signature should be as low as the one you measured ? Only such a reworking of the discussion will be able to convince the readers that the plume observed is originally from the sediment.
Citation: https://doi.org/10.5194/egusphere-2026-3486-RC2
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Dissolved and particulate Fe concentrations and Fe isotope compositions from the Peruvian upwelling zone (M136 to M138) G. Chen et al. https://doi.pangaea.de/10.1594/PANGAEA.995338
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- 1
This manuscript presents a valuable dataset of dissolved and particulate Fe concentrations and their isotopic compositions in the Peruvian oxygen minimum zone during a post-El Niño period. Through comparison among multiple cruises conducted by the authors, together with comparisons with previously published cruises, the study highlights how the El Niño-Southern Oscillation influence sedimentary Fe release and the subsequent offshore Fe flux. These observations provide valuable constraints on the temporal variability of Fe cycling in oxygen-deficient marine environments and its sensitivity to climate-driven changes in ocean circulation and redox conditions. The manuscript is generally well organized and clearly written. However, several aspects of the interpretation and proposed mechanisms would benefit from further clarification and discussion.
Major concerns:
(1) In Discussion 4.1, the authors found that dFe and pFe were transported farther offshore in the Peruvian OMZ during their sampling period than during the GP16 cruise, and attributed this difference primarily to variations in upwelling intensity. However, it remains unclear whether the dFe efflux from shelf sediments differed between the two periods. In addition, variations in horizontal current velocity between the two periods could also influence the offshore transport of Fe. Could the authors provide evidence or further discuss whether these factors differed between the two periods?
In addition, the 14°S section differs from the 12°S section, particularly in terms of the pFe/pAl ratios. The authors attribute this difference mainly to variations in sedimentary reducing dFe efflux between 12°S and 14°S, potentially related to differences in the width of the continental shelf. However, the maximum dFe concentrations along the 14°S section do not appear to be much lower than those at 12°S (e.g., TM94 vs. TM56), which may suggest that the sedimentary dFe efflux at 14°S is not necessarily lower. Could differences in the transformation of dFe to pFe also contribute to the observed contrast in pFe/pAl ratios? In addition, TM94 at 14°S is much closer to the continent. Could an additional lithogenic particulate Fe input contribute to the relatively lower pFe/pAl ratios? Lithogenic proxies, such as particulate Al or Ti, may help constrain this possibility.
Another possibility is that the relatively high pFe/pAl ratios along the 12°S section may partly reflect a biogenic Fe contribution. A pronounced secondary fluorescence maximum occurs near the oxic–anoxic interface, particularly at TM57 and TM56, where relatively high pFe/pAl ratios are also observed. Ohnemus et al. (2016) reported elevated trace metal concentrations in the ODZ during the GP16 cruise and suggested the presence of low-light, autotrophic communities. Biogenic proxies, such as particulate P and C, may provide useful constraints on this possibility.
(2) In Discussion 4.3, the authors suggest that the relatively heavy δ⁵⁶dFe and lighter δ⁵⁶pFe observed in the anoxic waters at TM66 and TM75 are related to ligand effects. Is there any additional evidence that the ligands in this depth range have distinctive characteristics that could explain their apparent influence on Fe isotope fractionation?
Furthermore, the authors discuss the potential influence of reductive and/or non-reductive dissolution of Peruvian slope sediments on deep water Fe. However, the dFe concentrations do not show an obvious increase in the deep waters, which raises questions about the extent to which Peruvian slope sediments influence the deep-water dFe pool. In addition, the authors state that “the δ⁵⁶dFe of −0.5‰ and 0‰ off Peru indicates both non-reductive and reductive release is important…” Could the authors clarify the sources of these two values? Is the −0.5‰ value the minimum δ⁵⁶dFe observed at TM75?
Minor comments:
Line 84: The original sentence does not explicitly indicate that M136, M137, and M138 are cruise names. We suggest adding this clarification, e.g., “…showing sampling stations from cruises M136, M137, and M138.”
Line 103: This sentence mentions “Class 5,” but it is unclear which standard or classification system it refers to. I suggest briefly specifying the relevant standard in parentheses after “Class 5” to improve clarity.
Line 123: Please clarify the source of the Fe(II) data and whether they were collected during the same period and at the same stations as those in this study.
Line 131: “…, while pFe varied from 1 to 17 nmol kg-1 (Figs. 2c and 3)”. The reported increase in pFe does not appear to be clearly supported by Figs. 2c and 3.
Line 140: Since the description of the particulate Fe data includes depth profiles at stations TM77, TM56, and TM75, as well as a few scattered data points at stations TM57, TM66, and TM58, the latter information should also be clarified in the first sentence.
Line 142: The full name of UCC should be provided when it first appears in the text.
Lines 150-155: (1) In Fig.3, the location and extent of the continental shelf in the last panel appear inconsistent with those in the first three panels, but all four panels share the same latitude values on the x-axis. Although the authors state that the location of TM56 in panels (a)–(c) is offset for clarity, it would be preferable to use the same transect/location consistently across all four panels. This would avoid potential confusion or misunderstanding when comparing the panels. (2) Please provide more explicit information. For example, the DO contours are shown with grey lines, this should be stated clearly in the figure caption. In addition, please clarify which cruise TM54 and TM60 belong to.
Line 175: A period (“.”) should be used instead of a comma (“,”) at the end of this sentence.
Lines 182-184: This sentence associates a δ⁵⁶pFe value of approximately −0.2‰ with the high pFe/pAl ratio at TM57. However, there seems no corresponding particulate Fe isotope data available for TM57. Could you please clarify the source of the −0.2‰ value? If direct data from TM57 are unavailable, perhaps the particulate Fe isotope data from TM56 could be used as a reference, given their similar characteristics.